2010-05-07_PERMIT FILE - C2009087 (34)

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width at an invert elevation of 6,776.6 feet. Side slopes are 2.5H: IV and 5.011: IV, respectively. The crest and spillway outslope will be armored with rock riprap in accordance with Rule 4.05.9(2)(a)(i). The calculated peak discharge from the 25 -year, 24 -hour storm event is 14.7 cfs (see Addendum 2.05.3- E2.D -2). The 25 -year, 24 -hour event peak stage is 6,777.0 feet. The top of the embankment is 6,780.0. Both the spillway crest and outslope will be protected with a rock riprap liner. The channel utility design (see Addendum 2.05.3- E2.E -3) was used to size the rock riprap. Rock riprap size was determined using the PADER (Pennsylvania Department of Environmental Resources, Stover, 1990) method. Given the uncertainty in determining riprap D50 for steep slopes with the PADER Method, SCCC adds an additional 3- inches to the model recommended D size. The D50 for the spillway crest is 4.0 inches; the D5o for the spillway outslope is 6- inches. The SEDCAD4 demonstration and as -built drawing (see Map 2.05.3 -E2.D, PECOCO Pond Spillway Re- Design) indicate there is a minimum of 1.0 feet of freeboard in the spillway as the design event runoff discharges through the spillway. Therefore, the pond as designed and constructed meets the intentions of Rule 4.05.9(7)(d). 5.0 Summary Most of the capacity of PECOCO Pond is attributed to highwall pit excavation. Because the pond is not a sedimentation structure it does not need to meet discharge effluent limitations and the intentions of Rule 4.05.2(7). A combination spillway has been constructed in accordance with Rule 4.05.9(2)(a)(i); the spillway is designed to safely pass the peak runoff from the 25 -year, 24 -hour • event. The impoundment was designed and constructed to be stable and provide adequate freeboard, therefore, the structure complies with the appropriate rules and regulations for an impoundment structure. Sage Creek Coal Company will periodically inspect the structure for sediment storage, structural integrity and hazardous conditions. Maintenance or remedial work will be performed in accordance with the approved mine permits. 6.0 Design Certification The impoundment and associated structures are pre - existing. The construction as -built drawings were prepared under the supervision of others. The remedial action plan design was completed by Seneca Coal Company and certified by Mr. Ronald J. Gerkhe, a Colorado registered professional engineer (P.E. No. 19959) experienced in the design and construction of earthen structures. The 2005 As -Built Report was certified by Mr. Robert J. Shevling, a Colorado registered professional engineer (P.E. No. 37382) experienced in the design and construction of earthen structures. I am a registered professional engineer in the State of Colorado experienced in the design, construction and operation of earthen structures. I have reviewed the design documents and inspected the site. Map 2.05.3 -E2.D, PECOCO Pond Spillway Re- Design, presents the impoundment configuration. To the best of my knowledge, based on my professional expertise, PECOCO Pond was designed and constructed in accordance with prudent state -of -the -art of earthen . embankment structures. 2.05.3 -E2.D5 12/09